ABSTRACT Plasmodium vivax causes approximately 100 million cases of clinical malaria each year and many more cases of asymptomatic parasitemia. Although not as virulent as P. falciparum, P. vivax nonetheless causes chronic anemia during infancy and life-threatening morbidities, including severe anemia, respiratory distress, renal failure and splenic rupture. After release from hepatocytes, merozoites only infect reticulocytes that in culture rapidly mature to erythrocytes, which limits in vitro culture and laboratory research. Unfortunately current vaccine candidates do not induce strong immunity in non-human primates. New insights are needed to develop an effective vaccine against P. vivax malaria. Previously immunological control of blood stage infection was thought to depend primarily on antibodies rather than on T cells, because mature red blood cells do not express HLA class I. Because reticulocytes, unlike mature erythrocytes, retain RNA and a high capacity for protein expression, we hypothesized that infected reticulocytes might act as antigen-presenting cells and targets for CD8+ T lymphocytes during malaria. Indeed, our preliminary studies, using samples from an endemic area of Amazonia, indicate a high frequency of HLA class I+ reticulocytes circulating in P. vivax-infected patients and a high proportion of activated circulating CD8+ T cells, which, when co-cultured with infected reticulocytes, form an immune synapse, secrete IFN?, release their cytotoxic granules and kill. Thus we hypothesize that CD8+ T cells recognize infected reticulocytes and that CD8 T cell immunity is an unappreciated, but important, component of P. vivax immunity. To test this hypothesis, in Aim 1 we will investigate whether infected reticulocytes present parasite antigens bound to HLA class I that are recognized by P. vivax-specific CD8+ T lymphocytes from malaria patients. In Aim 2 we will determine whether P. vivax-specific CD8+ T lymphocytes recognize and lyse infected reticulocytes by cytotoxic granule exocytosis and also kill intracellular parasites in a granulysin-dependent manner. The aim of these experiments is to confirm the preliminary data and begin to define how infected reticulocytes are recognized and destroyed by cytotoxic killer T cells. These data should open a new perspective for development of P. vivax malaria vaccines.